Reduced temperature and pressure powder metallurgy process for consolidating rhenium alloys

ABSTRACT

Pressure powder metallurgy process for consolidating refractory or rhenium alloys using a reduced temperature and elevated pressure. Rhenium metal has high temperature strength and wear resistance but has a very high melting point as a pure metal and thus is difficult to use as a coating for many alloys having lower melting points. The reduced temperature and elevated pressure alloying process of the rhenium allows it to be used as a coating for other metal alloys, such as nickel and steel alloys, providing some high temperature and wear resistance due to the properties of the rhenium material in the coating.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of Ser. No. 10/243,445 filed on Sep.13, 2002, now U.S. Pat. No. 6,821,313.

This patent application claims the benefit of U.S. ProvisionalApplications 60/284,737 for Reduced Temperature And Pressure PowderMetallurgy Process For Consolidating Rhenium Alloys and 60/384,631 forUse of Powdered Metal Sintering/Diffusion Bonding to Enable ApplyingSilicon Carbide or Rhenium Alloys to Face Seal Rotors, both filed on May31, 2002. This patent application is also related to U.S. patentapplication Ser. No. 10/138,090 filed May 3, 2002 for Oxidation and WearResistant Rhenium Metal Matrix Composite, now U.S. Pat. No. 6,773,663,and U.S. patent application Ser. No. 10/138,087 filed May 3, 2002 forOxidation Resistant Rhenium Alloys, now U.S. Pat. No. 6,749,803. All theforegoing applications are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates refractory metal alloys and to powder and othermetallurgy processes for consolidating rhenium alloys at reducedtemperatures and pressures so that rhenium alloys may be used ascoatings for alloys that have lower melting points than rhenium itself,such as steel and nickel alloys, as well as structural shapes where theentire object is made of rhenium alloy.

2. Description of the Related Art

In certain aerospace industry applications, a device's operatingefficiency can be increased by increasing the operating temperature forthe device. At these elevated temperatures, strength can be reduced andwear can accelerate for many materials. Wear resistant coatings existtoday, but the expense of applying them to conventional substratematerials, such as steel, nickel, and other conventional hightemperature alloys, reduces their cost effectiveness.

One group of materials that can have excellent wear rates is refractorymetals. However, they are expensive and heavy and so are generallyrelegated to use as coatings, as whole-part fabrication can be difficultand expensive. Some of such refractory metals have high temperaturestrength and/or adequate wear resistance but, because their meltingtemperatures are so much higher than the substrate materials, they canbe difficult to use.

Powder metallurgy can be used to fabricate and/or coat cheaper substratematerials with more wear-resistant coatings. Powder metallurgy can use aforming and sintering process for making various parts out of metalpowder. After a component has been generally shaped by forming ormolding sintering is a high temperature process that can be used todevelop the final material properties of the component. It can involveheating the powder to temperatures below the melting point of the majorconstituent in an inert atmosphere to protect against oxidation.Temperatures of approximately 80% of the melting point of the mainconstituent material can be used so as not to melt the component andaffect its shape. Popular raw materials used in the production of powdermetallurgy components are metal powders. These consist of fine, highpurity metal powders produced by processes such as atomization,pulverization, chemical reduction, electrolytic techniques or mechanicalalloying. Of these processes, atomization is a popular technique. In oneprocess, the metal powder is compacted by injecting it into a closedmetal cavity (the die) under pressure. This compacted material is placedin an oven and sintered in a controlled atmosphere at high temperatures,and the metal powders coalesce and form a solid. A second pressingoperation, repressing, can be done prior to sintering to improve thecompaction and the material properties.

Rhenium (chemical element symbol Re) is one such refractory metal thatis useable for powder metallurgy. It melts at 5,741° F. (3,172° C.,3,445° K) and consolidating it by powder metallurgy can occur atapproximately 3,272° F. (1,800° C., 2,073° K) and 20,000 to 30,000 psipressure. Since steel alloys melt near 2,700° F. (1,482° C., 1,755° K)and nickel alloys melt near 2,500° F. (1,371° C., 1,644° K),conventional powder metallurgy techniques generally are not suitable forcoating these metal substrates or any others with a melting temperaturebelow the consolidation temperature or with very low strength at thesetemperatures.

In view of the foregoing, there is a need for a cost effective, robustreduced temperature and/or pressure powder metallurgy process forrefractory metals that addresses one or more of the drawbacks identifiedabove. The present invention satisfies one or more of these needs.

SUMMARY OF THE INVENTION

This invention dramatically reduces the temperature and pressurerequired for consolidation of rhenium by including lower temperatureconstituents that have full or partial solubilities with rhenium. Theseadditions contribute to consolidating the individual rhenium particlesto each other most likely through enhanced diffusion and deformabilityat the particle interface. As a result, the cost is reduced, making thematerial's use more cost effective. In addition, these constituents canenhance the oxidation resistance of the alloy. The oxidation resistanceand an application for face seal and ceramic encapsulation is describedin more detail in incorporated by reference U.S. patent application Ser.Nos. 10/138,090 and 10/138,087. The alloy elements used to date includecobalt, nickel, chromium and manganese. This approach has succeeded andenabled the coating of steel discs for use in face seals as noted in theprovisional application referenced above.

An exemplary pressure powder metallurgy process for creating a rheniumalloy may include the steps of providing a first powder of a refractoryalloy, providing a second powder of at least a second metal, the secondmetal being partially or fully soluble with the refractory alloy, mixingthe refractory alloy and second powder to provide a mixture, and heatingthe mixture under pressure to fuse the refractory alloy with the secondpowder so as to form an alloy in a manner that high temperatures may beavoided, yet the refractory alloy and second powders may be fused.

An exemplary refractory metal alloy produced by the present inventionmay include rhenium, where rhenium comprises the largest constituent ofthe alloy by atomic weight, and a metal selected from the groupconsisting of cobalt, chromium, manganese and nickel.

Another exemplary metal alloy produced by the present invention mayinclude at least 50% by atomic weight refractory metal, and a metalselected from the group of cobalt, chromium, manganese and nickel.

Other features and advantages of the present invention will becomeapparent from the following description of the preferred embodiment(s),taken in conjunction with the accompanying drawings, which illustrate,by way of example, the principles of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The detailed description set forth below in connection with the appendeddrawings is intended as a description of presently—preferred embodimentsof the invention and is not intended to represent the only forms inwhich the present invention may be constructed and/or utilized. Thedescription sets forth the functions and the sequence of steps forconstructing and operating the invention in connection with theillustrated embodiments. However, it is to be understood that the sameor equivalent functions and sequences may be accomplished by differentembodiments that are also intended to be encompassed within the spiritand scope of the invention.

As previously stated, popular raw materials used in the production ofpowder metallurgy components are the metal powders. These consist offine, high purity metal powders produced by processes such asatomization, pulverization, chemical reduction, electrolytic techniquesor mechanical alloying. Of these processes, atomization is one populartechnique. Lubricants may be added to the metal powders to reducefriction between powders and pressing dies, which in turn reducespressure gradients. The raw materials are formed into shapes usingpressure-based techniques such as cold uniaxial pressing, metalinjection molding, cold isostatic pressing, hot isostatic pressing orhot forging. The latter two processes also combine the forming andsintering processes into a single process.

Compaction of the raw materials at high pressures forms apreliminarily-shaped component, also called a “green shape”, where theparticles bond together by mechanical interlocking and/or cold welding.Pressing at sufficiently high pressures gives the green shape enoughstrength to be handled and machined. The amount of compressionexperienced by the powder during the forming process will tend to relateto its green density. This in turn can control the amount ofshrinkage/growth the powder compact will undergo during the sinteringprocess. It will also influence the physical properties of the finalcomponent. Sintering is a high temperature process used to improve themechanical properties of the component. It can involve heating totemperatures below the melting point of the major constituent in aninert atmosphere to protect against oxidation. During the sinteringprocess, adjacent particles bond together by solid state diffusionprocesses. As mentioned earlier, processes such as hot isostaticpressing (HIP) and hot forging combine the sintering and forming processinto a single stage.

Sometimes secondary operations are used to improve surface finish ormake sure components fall within required tolerances. Such operationsmay include, milling, drilling, grinding, repressing. Density andstrength can also be increased by a secondary sintering and/orrepressing operations. Sometimes, porous metal components aredeliberately produced so that they can be infiltrated with secondarymaterials. Further, components can be produced such that they haveinterconnected pores, or capillary pores. These can be infiltrated withoil or other lower melting point metals. For this to work properly,oxide free pores may be required, which could be produced by processingin an inert atmosphere.

The HIP process provides a method for producing components from diversepowdered materials, including metals and ceramics. During themanufacturing process, a powder mixture of several elements is placed ina container, typically a steel can. The container is subjected to anelevated temperature and a very high vacuum to remove air and moisturefrom the powder. The container is then sealed and HIP'ed. Theapplication of high inert gas pressures and elevated temperaturesresults in the removal of internal voids and creates a strongmetallurgical bond throughout the material. The result is a cleanhomogeneous material with a uniformly fine grain size and a near 100%density. The reduced porosity of HIP'ed materials enables improvedmechanical properties and increased workability. The HIP processeliminates internal voids and creates clean, firm bonds and fine,uniform microstructures. These characteristics generally are notpossible with conventional welding or casting. The substantial reductionor virtual elimination of internal voids enhances component performanceand improves fatigue strength. The process also results in significantlyimproved non-destructive examination ratings.

One alloy, designated Honeywell Alloy 30, has a nominal composition of20% cobalt, 15% chromium, 5% manganese and 60% rhenium. Another alloy,HRA 33, has a composition of 20% cobalt, 10% nickel, 10% chromium, and5% manganese with the balance of 55% being rhenium. Ceramic materialscan be encapsulated in the alloy HRA 35 with a composition of 15%silicon carbide (SiC), 10% nickel (Ni), 10% cobalt (Co), 10% chromium(Cr), 5% manganese (Mn), and 50% rhenium. The percentages set forthherein are generally atomic percentages.

These alloys have been consolidated at temperatures as low as 1,000° C.(1,832° F., 1,273° K) and as high as 1,800° C. (3,272° F., 2,073° K),and pressures as low as 250 psi and as high as 14,000 psi. However, atemperature of 1,000° C. (1,832° F., 1,273° K) and a pressure between2000 and 3000 psi results in almost zero porosity and little if anydeformation of substrates. Typically, the temperature must be somewhathigher for lower pressures.

Likewise, when the pressure is higher, the temperature may be lowered.In summary, the range of at least partial consolidation is bounded onthe lower end of pressure by 250 psi. To date, the lowest limit oftemperature tried by the inventor is 1,000° C. (1,832° F., 1,273° K) buta preferred temperature is 1,100° C. (2,012° F., 1,373° K). Lowertemperatures may work but have not yet been tried. The upper limit oftemperature is generally bounded by the element with highest vaporpressure and the strength of the substrate at the temperature ofinterest. For example, at 1,500° C. (2,732° F., 1,773° K) large voidsare quickly formed internally in the chromium areas, most likely due tochromium vaporization. In addition, since many steel alloys losestrength rapidly as the temperature approaches 1,200° C. (2,192° F.,1,473° K), the part is deformed even with relatively low pressures ifthe temperature is too high.

The best results have been achieved with rhenium combined with multiplealloy constituents, but are also achievable with only chromium, nickel,cobalt or manganese added individually to pure rhenium. The alloycombinations with a rhenium base assist in oxidation resistance.

In operation, a pressure powder metallurgy process for creating arhenium alloy includes providing a first powder of rhenium, providing asecond powder of at least a second metal, the second metal beingpartially or fully soluble with rhenium, mixing the rhenium and thesecond powder to provide a mixture, and heating the mixture underpressure to fuse the rhenium with the second powder so as to form analloy where temperatures significantly below 80% of the melting point ofrhenium may be used, yet the rhenium and the second powder may be fused.The second powder in the process may be any one of cobalt, chromium,manganese nickel, or various combinations of these metals, depending onthe intended use of the final product. In addition, ceramic materialssuch as silicon carbide may also be added to the second powder tocontrol desired properties, for example, insulation properties, thermalconductivity, and/or electrical conductivity, among others.

While the present invention has been described with reference to apreferred embodiment or to particular embodiments, it will be understoodthat various changes and additional variations may be made andequivalents may be substituted for elements thereof without departingfrom the scope of the invention or the inventive concept thereof.

Particularly, other refractory metals may be substituted for rhenium.One such metal is tungsten (chemical symbol W). The processes disclosedherein may be adapted for other refractory materials. Generally, thealloys and materials disclosed herein have a refractory metal as a, orthe, major constituent and may generally have a refractory metal, suchas rhenium or tungsten, as the majority constituent.

In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to particular embodiments disclosedherein for carrying it out, but that the invention includes allembodiments falling within the scope of the appended claims.

1. A pressure powder metallurgy process for creating a rhenium basedalloy, the process comprising the steps of: mixing a first powder and asecond powder to provide a mixture, the first powder comprising rheniumand the second powder comprising cobalt, chromium, and manganese,wherein the mixture comprises at least 50% by atomic percent of rheniumand at least 25% by atomic percent of cobalt, chromium, and manganese;and heating the mixture under pressure to fuse the rhenium with thesecond powder so as to form the rhenium based alloy.
 2. A process forcreating an alloy as set forth in claim 1, further comprising: heatingthe mixture to a temperature between approximately 1000° C. and 1800° C.3. A process for creating an alloy as set forth in claim 2, furthercomprising: heating the mixture under a pressure of betweenapproximately 250 psi and 14000 psi.
 4. A process for creating an alloyas set forth in claim 3, whereby the second powder further comprises oneor more materials selected from the group consisting of nickel andceramic.
 5. A process for creating an alloy as set forth in claim 3,whereby the second powder further comprises nickel.
 6. A process forcreating an alloy as set forth in claim 3, whereby the second powerfurther comprises silicon carbide and nickel.